Clonotypic IgM transcripts are detectable in bone marrow and blood of WM patients. Single-stage RT-PCR of unfractionated mononuclear cells prepared from BM and peripheral blood as described in “Patients and methods.” An equal amount of cDNA was used in each sample. Negative control PCR reactions without cDNA or with cDNA from BM or blood of healthy controls (n = 2) or unrelated WM patients (n = 8) lacked bands. Water controls were negative. Amplification was performed using (A) CDR2 (patient specific) and CDR3 (patient specific) primers or (B) CDR3R (patient specific) and CμS (S) or CμM (M) primers.